The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008
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(Volpe, 2003). In fact, the number of GCP’s down to 4 / 5 did
not decrease the accuracy significantly and sub-meter accuracy
can be achieved using only a few GCP’s and the RPC models
(Baltsavias et al., 2006; Fraser et al, 2006; Michalis and
Dowman, 2006). Besides, a well spread distribution of even a
few GCP’s is more beneficial to accuracy improvement than a
dense but poorly spread distribution (Li et al., 2000).
In this study, 12 GCP’s were established and marked in May
2002 before recording of images. The coordinates of these
GCP’s have been measured with Global Positioning System
(GPS). These GCP’s have been used with different distributions
in the orthophoto productions.
2.3 Images
Stereo-pair IKONOS (dated August 4 th , 2002), mono Quickbird
(dated May 26 th , 2002) and 1:16.000 scaled aerial images (dated
August 29 th , 2002) were provided by Inta Inc. (Turkey),
Eurimage (Italy) and General Command of Mapping (Turkey)
free of charge. The information about images are in Table 1.
lfcoo-3 (Pas-1)
Nirengi-2
Pa$2
❖
Pas-30
❖
Pas-22
❖
313
A (Pas-5) ^
Pas-€003
❖ A
P as-20 m
Pas-9
❖
Pas-7
P as-8 ^
o
312
A
Pas-12
❖
Pas-29
❖
Pas-13
❖
1308
A
Pas-24
❖
302
A
Nirengi-3-lNirengi-3
❖ ❖
309
Pas-25
❖
317
A
1315
A
3(3
Jl
Pas-15Pas-27
❖ ❖
Pas-16
t^as-17)
o
Niren» 1 '
❖ Q
Nominal
Collection
Azimuth
Nominal
Collection
Elevation
Sun Angle
Azimuth
Sun Angle
Elevation
IKONOS-1
343.397 0
79.1812°
145.587°
64.083 °
IKONOS-2
206.640 0
65.7324 0
146.031 °
64.182°
Quickbird
239.034 0
83.0699 0
139.486°
67.278 °
Table 1. Azimuth and elevation angles of satellite images
Figure 1. Distribution of GCP’s (A) and CP’s (O)
The last DEM has been produced from IKONOS stereo images.
8 GCP’s and 34 tie points were used in this process and a
correlation success percent of % 97.2919 was obtained after
production. It can be accepted that the accuracy of IKONOS-
DEM is approximately ± 2 m (Erdogan, 2008).
1:16.000 scaled 40 stereo aerial images have been taken in 5
strips (in east-west and west-east direction) using kinematic
GPS technique. The overlaps between the images and the strips
were % 60 and % 30 respectively.
2.4 Check Points (CP’s)
Easily-recognizable and well-distributed 35 CP’s such as road
junctions and field comers were selected from orthophotos and
mosaic images. However, because of the time difference
between images and land measurements, the weather conditions
and the travel restrictions, only 27 CP’s coordinates could be
measured with GPS (Figure 1).
The field applications have been realized during two days in
March 2003 using two reference points.
2.5 Digital Elevation Models (DEM’s)
Photogrammetric, cartographic and Ikonos DEM’s are used for
different orthophoto productions.
The photogrammetric DEM was produced by collecting data
manually from 1:16.000 scaled aerial images in 20 m intervals
(i§can et al., 2004). It is considered that the accuracy of
photogrammetric DEM is approximately ± 1 m.
The cartographic DEM has been already produced by General
Command of Mapping using printed sheets in 1:25.000 scale. In
this production, the printed sheets have been scanned first and
then the contour lines have been digitized through semi
automatic methods. It can be considered that the accuracy of
cartographic DEM is approximately ± 5 m.
3. GEOMETRIC ACCURACY ASSESSMENT
For geometric accuracy assessment, orthophotos from satellite
images and mosaic images from aerial photographs have been
produced first. Then, check points (CP’s) have been selected
from both orthophotos and mosaic images and finally, ground
and image coordinates were measured and compared.
3.1 Orthophoto and Mosaic Production
Using photogrammetric, cartographic or Ikonos DEM, various
ortho and mosaic images have been produced with different
GCP’s distributions. Minimum 4/5 GCP’s and maximum 7-11
GCP’s have been used in ortho and mosaic productions.
Furthermore, a stereo IKONOS image (using 7 GCP’s and 91
tie points) and orthophotos without GCP’s were also produced
using direct sensor orientation. In orthophoto productions from
satellite images, the rational polynomial coefficients (RPC’s)
approach has been applied with a polynomial third order
refinement.
3.2 Coordinate Measurements
Planimetric coordinates of the CP’s have been measured on all
orthophoto/mosaic images and stereo IKONOS model. These
coordinates have been compared with GPS coordinates.
Because of big time difference between images and
measurements, some difficulties have been encountered in
detection of features on images and in finding the relevant CP’s
in field. Therefore it has been tried to find the pixels which
have the same grey scale values and represent the same features.
In this process, it was very beneficial to use the brightness or
darkness of adjacent pixels as described in Helder et al. (2003).